Metal Rolling

Metal Rolling [01] Definition ROLLING is the process of reducing the thickness or changing the cross section of a long work-piece by compressive forces applied through a set of opposed rotating rolls. This Primary Working operation takes a solid piece of m etal (generally from a cast state, such as an ingot) and breaks it down successively into shapes such as slabs, plates, and billets. Rolling is a bulk deform ation process. The term bulk deformation is applied to the processing of workpieces having a relatively small surface area-to-volume (or surface area-to-thickness) ratio. In all bulk deform ation processing, the thickness or cross -section of the workpiece changes. The other three basic bulk-deformation processes for metals are forging, extrusion, and drawing of rod and wire. Temperature, size, and shape of the workpiece can group rolling processes. For example, using temperature as a criterion, the categories would be hot rolling and cold rolling. If we are interested in size, it is important to note that plates are generally regarded as having a thickness greater than 6 mm (1/4 in.) whereas sheets are generally less than 6 mm thick. Flat Rolling, strip rolling, or simply rolling, is the most basic operation, where the rolled products are flat plates and sheets, and the m ain purpose is to reduce the thickness of the material. This process results in the production of flat plate, sheet, and foil in long lengths, at high speeds, and with good surface finish, especially in cold rolling. It requires high capital investment and low to m oderate labor cost.

[02] Classification of Rolling Processes Rolling produces many products; including flat sheet, I – beams, round bars, and tubular products. Steel & Aluminum are the most commonly rolled materials. Classification based on Temperature: Hot rolling & cold rolling Classification based on Shape: Flat rolling & Shape rolling Classification based on Size of Flat rolling: Slabs [A>16”] continued by further rolling to plates [> 6mm] and sheets [<6mm] Billets [1.5”x1.5” to 6”x6”] continued by further rolling to rods & bars Blooms [> 6”x6”] continued by further form rolling into sections Classification based on Rolling Mills (Stands): 2-high, 2-high reversible, 3-high, 4-high, cluster, planetary, and Tandem rolling m ills Other Rolling Processes: Ring Rolling, Thread Rolling, Mannesmann Rolling, etc.

[03] Hot Rolling Rolling is first carried out at elevated tem peratures (hot rolling), where the coarse-grained, brittle, and porous structure of the ingot or continuously cast metal is broken down into a wrought structure, with finer grain size. Temperature ranges for hot rolling are similar to those for forging, with 925-1250 0C for Alloy steels. Changes in the grain structure of cast or large-grain wrought metals during hot rolling. Hot rolling is an effective way to reduce grain size in metals for improved strength and ductility.

Cast structures of ingots of continuous castings are converted to a wrought structure by hot working.

[04] Cold Rolling Cold rolling is the process of rolling at low temperatures. It is usual applied as finishing process after hot rolling to enhance strength and hardness, and ensure high surface quality. During cold rolling, annealing may take place to facilitate further cold rolling. C old rolling is the process of rolling at low temperatures. It is usual to roll very large quantities of material as long uninterrupted coils. The loads must remain steady, therefore it is common for several rolling mills to work in tandem, and it is essential that the tension in the strip between mills be held within close limits to maintain an even gauge. As the thickness of the slab is reduced, its length increases and the speed of the outgoing strip may reach values up to about 5,000 ft/m in. One benefit of cold rolling is cold working, which increases the strength of a product.

[05] Flat Rolling The most basic operation in rolling is flat rolling, that produces flat plates and sheets, which are used in applications such as ship hulls, and nuclear vessels, as well as food containers and Alum inum foil. A rolled sheet may not be sufficiently flat as it leaves the roll gap because of variations in the material or in the processing parameters during rolling. To improve flatness, the strip is then passed through a series of leveling rolls.

[06] Shape Rolling Shape rolling involves the production of various structural shapes, such as I-beams, at high speeds. In general, it requires shaped rolls and expensive equipm ent, low to moderate labor cost and moderate operator skill.

[07] Ring-rolling In ring rolling, a small-diameter thick ring is expanded into a larger-diam eter, thinner ring with a desirable cross-section. The ring is placed between two rolls, one of which is driven, and the other of which is idle. The advantages of ring rolling, compared with other processes for making the same part, are short production runs, material savings, close tolerances, and favorable grain flow direction. Typical applications of ring rolling are large rings for rockets and turbines, gearwheel rims, ball and roller bearing races, flanges and reinforcing rings for pipes, and pressure vessels.

[08] Thread rolling In the cold-forming thread and gear rolling process, threads are formed onround rods or work pieces by passing them between reciprocating or rotating dies. Typical products include screws, bolts, and similar threaded parts. Production rates depend on the diameter of the product. With small diameters therates can be as high as eight pieces per second and with larger diam eters (as much as 25mm) about one per second. The thread rolling process generates threads without any metal loss and with greater strength because of cold working. The surface finish is very smooth, and the process induces com pressive residual stresses on the surfaces, which improves fatigue life. This process is used in the production of almost all externally threaded fasteners.

[09] Rolling Mills Rolling Mill (Stand): In rolling, a squeezing type of deformation is accomplished by using two work rolls rotating in opposite directions. The principal advantage of rolling lies in its ability to produce desired shapes from relatively large pieces of metals at very high speeds in a som ewhat continuous manner. The schematic sketch illustrates simply the main components of rolling mill.

Working Rolls: Deforms the workpiece. Chucks: As rolls back supporting. Load cell: To me asure the rolling load. Screw: To adjust the rolling gap. Housing: Rigid machine frame.

Mills are classified by descriptive dimensions that indicate the size of the m ill, by the arrangement of roll stands, and by the type of product that is rolled. The dim ensions used to indicate size vary depending on the type of mill and the product. However, there are three principal types of rolling mills, referred to as two-high, three-high, and four-high mills. This classification, as the names indicate, is based on the way the rolls are arranged in the housings. A two-high stand consistsof two rolls, one positioned directly above the other; a three-high mill has three rolls, and a four-high mill has four rolls, also arranged one on top of the other. Two-high mills may be either pull-over (drag over) mills or reversing mills. In pull-overtype mills, the rolls run in only one direction. The workpiece must be returned over the top of the mill for further Reversing mills em ploy rolls on which the direction of rotation can be reversed. Rolling then takes place alternately in two opposite directions. Reversing m ills are am ong the m ost widely used in industry, and can be used to produce slabs, blooms, plates, billets, rounds, and partially formed sections suitable for rolling into finished shapes on other m ills.

Two-high mills Three-high mills: In three-high mills, the top and bottom rolls rotate in the sam e direction, while the m iddle roll rotates in the opposite direction. This allows the workpiece to be passed back and forth alternately through the top and m iddle rolls and then through the bottom and middle rolls without reversing the direction of roll rotation. Four-high mills: Four-high mills are used for rolling flat material such as sheet and plate. This type of mill uses large backup rolls to reinforce smaller work rolls, thus obtaining fairly large reductions without excessive amounts of roll deflection. Four-high mills are used to produce wide plates and hot rolled or coldrolled sheet, as well as strip of uniform thickness.

Special Mills: Two other types of mills that are used are cluster mills and planetary mills.

The most common type of cluster mill is the Sendzimir m ill. In a typical Sendzimir mill design (Fig. 22a), each work roll is supported through its entire length by two rolls, which in turn are supported by three rolls. These rolls transfer roll-separating forces through four large backup rolls to a rigid, cast steel housing. Sendzimir mills are used for the cold rolling of sheet and foil to precise thicknesses. Planetary mills were developed in Germany to reduce slabs to hot-rolled strip in a single pass. This is accomplished by the use of two backup rolls surrounded by a number of small work rolls (Fig. 22b). Planetary mills are capable of reductions of up to 98 in a single pass, and have been designed up to 2030 mm (80 in.) in width. Planetary mill, used to accomplish large reductions in a single pass

[10] Rolling Defects Rolling defects can be classified as surface defects, shape defects, and internal or structure defects. Surface defects are scale, rust, scratches, gouges, pits, and cracks, which can be caused by inclusions and impurities in the original material, or due to improper material preparation. The most common rolling shape defects are: Wav y Edges: due to improper roll cam ber leads to more elongation at edges. Zipper Cracks: result of poor m aterial ductility at the rolling tem perature. Edge Cracks: result of poor m aterial ductility at the rolling temperature. Alligatoring: Com plex phenomenon caused by non-uniform deform ation and due to improper material structure (such as high sulfur contents in steels)

Residual stresses are due to non-uniform deformation. A) Residual stresses developed in rolling with small rolls or at small reductions in thickness per pass. B) Residual stresses developed in rolling with large rolls or at High reductions in thickness per pass.

[11] Analysis of Sheet Rolling

[1] Velocities in Sheet Rolling The rolls transfer energy to the strip through friction. As the strip of thickness h0 is dragged by the rolls into the gap between them , it decreases in thickness while passing from the entrance to the exit. Finally exiting with a thickness hF. Meanwhile its speed gradually increases from V0 at the entrance to VF at the exit. Under regular rolling conditions the strip m oves slower than the sur face speed of the rolls at the entrance to the gap between the rolls (V0 < VR). To keep the volume rate of metal flow constant, the velocity of the strip must increase as it moves through the roll gap, resulting in a final speed faster than the rolls (VF > VR) at the exit. Because VR is constant along the roll gap, sliding occurs between the roll and strip.

[2] Neutral Point There is a neutral point in between the entrance and the exit at which the speeds of the strip and the rolls are equal (V = VR). It is known as the neutral point or no-slip point. To the left of this point, the roll moves faster than the workpiece, and to the right the workpiece m oves faster than the roll. Because of friction at the interfaces, the frictional forces-act on the strip surfaces. The friction force acting along the surfaces of the rolls between the entrance and the neutral point advances the strip between the rolls, while the friction force acting between the neutral point and the exit opposes the rolling action. The difference between the friction on the entrance side and the friction on the exit provides the necessary power for rolling. The position of the neutral point is autom atically determined by the power required to deform the strip and to overcome frictional losses. The larger the reduction attem pted the farther the neutral point moves toward the exit, increasing the net friction drag force. The maxim um reduction possible occurs when the neutral point reaches the exit. After this point the process becom es unstable, and the rolls will skid over the strip and the strip will stop altogether. Forward slip in rolling is defined in terms of the exit velocity of the strip VF and the surface speed of the roll V R as: Forward slip = (VF- VR) / VR, and is a measure of the relative velocities involved.

[3] Friction In rolling, although the rolls cannot pull the strip into the roll gap without some friction, forces and power requirements rise with increasing friction. In cold rolling, the coefficient of friction m usually ranges between 0.02 and 0.3, depending on the materials and lubricants used. The low ranges for the coefficient of friction is obtained with effective lubricants and regim es approaching hydrodynamic lubrication, such as in cold rolling of aluminum at high speeds. In hot rolling, the coefficient of friction may range from 0.2, with effective lubrication, to as high as 0.7, indicating sticking, which usually occurs with steels, stainless steels, and high temperature alloys.

[4] Rolling Force and Power requirement The rolls apply pressure on the material to reduce its thickness. The normal perpendicular force F acting on the arc of contact can be estimated from the formula:

F= L W Yav e . Where L= length of arc of contact L = R (h0 – hF) W= the width of the strip Yav e = the average true yield stress of the strip

Assuming that the Force F acts at the middle of the arc of contact; Torque per roll = F* a Where a =L/2 Consequently the power required per roll = 2 πN * Torque per roll Power = 2(rolls) * [2π N * Torque per roll] Power = (2 π N F L) / (60 * 1000) Power = [(2 π N F L) / (60 * 1000)] * (1 / 0.745)

KW HP

[12] Selection of Manufacturing Process: (The Seven Principles)

Criteria to use in process selection: Material: Rolling is used primarily for m etals such as steel, alloys, and aluminum. These materials have high yield strength. Geometry: Since there are several types of rolling methods, the range of dimensions in rolling is extremely wide. The product can be as wide as 5 m (200 in.) and as thin as 0.0025 mm (0.0001 in.). The most common shapes are ingots and thin sheets and plates whose lengths are much larger than its width. Tolerance: By reducing the material's thickness in stages, very low tolerances are possible (in the order of 0.0001 in.) Surface finish is very smooth and can be reduced by the use of leveling rolls. As opposed to hot rolling, whose roll surfaces are generally rough and require notches and grooves to pull the m etal through, rolls in cold rolling are ground to a fine finish and are polished when used for special applications. Life Expectancy: Life expectancy for the components is high. Threaded com ponents contain compressive residual stresses on the surface, which improves fatigue life. The equipment will itself be durable with proper lubrication, though it is subject to roll deflection and flattening. Common roll materials are cast iron, cast steel, and forged steel, each having high strength and resistance to wear. Production Volum e: Because rolling is a fast, continuous process, it is most economical for a large production volum e. Level of Automation: Rolling mills can be highly automated with rolling speeds as high as 25 m/s (5000 ft/min.) In thread and gear rolling, production rates as high as 80 pieces per second are possible Cost: Rolling is usually a primary working operation, which m ay require additional working, including rolling.

References: Lecture notes of Dr. Ahmed Fareed, Ain shams University,2007.